glibc 2.34 has a hard break where you cannot compile with 2.34 and have it work with older glibc versions even if you use those version headers. It will always link __libc_start_main@GLIBC_2.34 (it's some kind of new security hardening measure, see https://sourceware.org/bugzilla/show_bug.cgi?id=23323).
Since additionally you also need to build all your dependencies with this same trick, including say libstdc++, it's really easiest to take GP's advice and build in a container with the old library versions. And nothing beats being able to actually test it on the old system.
Since additionally you also need to build all your dependencies with this same trick, including say libstdc++, it's really easiest to take GP's advice and build in a container with the old library versions. And nothing beats being able to actually test it on the old system.